Creating half-cylinder walls in a mesh
Another solution using vertex bevel.

Add an edge top and bottom (for now you can F a temporary face for your object, it will be deleted later)

Select the vertices that define the edges and press Ctrl + Shift +  B followed by P to change the profile.




Hide everything except the bevel you just created with H

Alt + Shift select and use Bridge Edge Loops to complete the shape on each side